A comparison between soft tissue and bone registration techniques for prostate radiotherapy Richard Small, Paul Bartley, Audrey Ogilvie, Nick West and Dr. John Frew Background Discussion in various studies including Owen et al highlight that the prostate gland moves independently of the pelvic bones in patients positioned supine for radiotherapy treatment[1]. Currently at Northern Centre for Cancer Care (NCCC) a bony registration technique is used to determine offset displacements. Study Aims Assess the difference in isocentric displacement between bone and soft tissue image registration. To ensure interobserver reliability is maintained when comparing soft tissue to bony matching which is currently used as the department standard. Method A CT-on-rails primatom was used to acquire the images. Adaptive targeting software (fig.1) provides bone match displacement values by comparing the treatment and localisation CT scans. Fig. 1. shows the outline of the planned CTV contour placed over the treatment CT scan thus allowing soft tissue displacements to be calculated.

Variation between operators SoftBoneSoftBone LatLongVertLatLongVertMag StdDev Mean Max Results Table 1 shows an impressive correlation between users looking at soft tissue images with an improvement when compared to bone matching. Graph 1 shows 623 of the 704 shifts made were within ±1mm when comparing the displacement gained via the 2 matching methods. The graph also demonstrates that there can be large differences between the 2 systems of up to 8mm. A comparison between soft tissue and bone registration techniques for prostate radiotherapy Fig.1 Table 1 Richard Small, Paul Bartley, Audrey Ogilvie, Nick West and Dr. John Frew

Conclusion + Recommendation A comparison between soft tissue and bone registration techniques for prostate radiotherapy With smaller variation between observers, a soft tissue matching technique is a viable option. For a high proportion of patients bony matching is an adequate form to determine displacements. Largest displacements predominantly in the vertical direction relating to bladder and rectal filling. Results suggest patients with variable rectal filling would benefit from soft tissue registration. In an era of IMRT and steep dose gradients we believe soft tissue image registration should be strongly considered as an accurate on-set treatment verification tool. Reference Owen R, Kron T, and Foroudi F. Comparison of CT on rails with electronic portal imaging for positioning of prostate cancer patients with implanted fiducial markers. Int. Rad. Onc. 2009; vol 74 issue Graph 1 Richard Small, Paul Bartley, Audrey Ogilvie, Nick West and Dr. John Frew